Interior permanent magnet machines have been widely chosen as driving and generating machines for various applications, including hybrid electric vehicles. Internal permanent magnet (IPM) electric machines have magnets built into the interior of the rotor. Typically, each magnetic pole on the rotor is created by putting permanent magnet (PM) material into one or more slots formed in the laminated stack of the rotor. Although slots are formed for the magnets, the slots are typically not completely filled with magnetic material. In particular, the slots are typically longer than the magnets, and the magnets are placed in the center portion of each slot. This results in a slot with a magnet in the center and voids at two opposite ends of the slot.
There are several reasons for shaping slots longer than the magnets such that voids are provided at the ends of the slots. One reason relates to the performance of the electric machine. If the PM material filled the slot, the ends of the magnet would tend to short circuit, making ineffective use of the PM material. Also, magnetic design for the flux in the steel that makes up the rotor favors smooth curve ends on the slot, rather than sharp corners as are typically provided at the ends of the magnets.
At least one other reason for shaping elongated slots relates to manufacturing issues. For example, to reduce manufacturing costs, it is generally desirable to minimize the amount of finish grinding on the magnet blocks. With elongated slots, only the sides of the magnet that will fit closely against the walls of the lamination stack within the slot need to be ground. Finish grinding is generally not required on the two magnet sides that face the slot ends. However, if the magnet was to fit closely with all sides of the slot, then grinding would be required on all sides of the magnet in contact with the lamination stack. In addition to the above, the additional room provided by the elongated slots makes it easier to insert the magnets into the slots during manufacturing.
Because of this rotor arrangement of curved slot ends and magnets that are shorter than the slots, some feature must be utilized to maintain the proper position of the magnets within the slots. Without such stabilizing features, the magnets would slide between the voids at the end of the slots. However, the use of stabilizing features in an IPM machine can affect the magnetic fields within the machine. If care is not taken in the design of the IPM machine, situations can occur where the magnetic fields can demagnetize the permanent magnets over time. Irreversible demagnetization of the permanent magnets can lead to decreased performance of the electric machine.
Accordingly, it would be desirable to provide stabilizing features for an IPM machine. It would also be desirable if such stabilizing features did not adversely affect the magnetic fields and related magnetic properties in the electric machine which might lead to demagnetization of the machine's permanent magnets. Furthermore, it would be desirable if such stabilizing features could be provided at a relatively small increase in manufacturing costs.
It would be desirable to provide a permanent magnet electric machine that provides one or more of these or other advantageous features as may be apparent to those reviewing this disclosure. However, the teachings disclosed herein extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the above-mentioned advantages.